In the art of glass fabrication involving the bending of a glass sheet into a particular desired configuration or curvature, it has long been known to use a hot press forming process. Such process can be used, for example, in the manufacture of curved motor vehicle windows. In a hot press forming process a glass sheet is heated in a lehr, typically as the glass passes over horizontally mounted ceramic rollers. In certain advanced versions of the process the glass is picked up by a so-called air suspension block ("ASB") at the end of the line of ceramic rollers. By a careful balance of vacuum and blown air, the ASB suspends the glass in a horizontal plane a few millimeters below the ASB surface. The glass at this point is still travelling through the lehr. At the end of the ASB suspension of the glass is terminated such that it drops onto a hot press forming tool. The hot press forming tool typically provides a support surface adapted to contact only the peripheral portion of the lower surface of the glass. To stop the forward motion of the glass and properly position it over the hot press forming tool, it is known to provide "nesters," a set of rotatable metal plates forming a "nest" in a horizontal plane above a hot press forming tool.
Each of the rotatable metal plates which form the glass-positioning nest at the end of the ASB above the hot press forming tool sits atop a metal post. Typically, such metal posts are mounted to mounting brackets fixed at spaced peripheral locations about the glass support surface of the hot press forming tool. The leading edges of the advancing glass sheet contact the outer peripheral surface of the metal plates. In fabricating a motor vehicle window, for example, three rotatable plates can be used to stop and position the glass sheet. As the window advances into the nest formed by the rotatable plates, it is guided by the rotatable plates into proper lateral position and is stopped in proper longitudinal position. With the glass sheet now properly positioned at the end of the ASB, the ASB vacuum is shut off and the hot glass sheet drops onto the hot press forming tool.
Primarily because of the extremely high operating temperatures within a lehr, the rotatable metal plates frequently do not rotate sufficiently freely on the metal posts on which they sit. High temperatures cause the metal plates to bind with the metal posts. When the advancing glass sheet contacts a metal plate which is not freely rotatable, it tends to pivot about the contact point, rather than being nudged laterally into proper location. If the hot glass sheet is not properly positioned by the rotatable metal plates, it will not drop into proper position on the press forming tool and will not be properly formed during the press forming process. A particularly difficult problem in this regard has been the occurrence of "galling" of metal parts which move over one another at such elevated temperatures. Galling includes pitting and flaking of the metal surface and typically causes binding of otherwise freely movable parts. This results in reduced productivity, reduced output and increased manufacturing costs.
It is an object of the present invention to provide a roller cylinder assembly of new and improved design. It is an object of certain preferred embodiments of the invention to provide a roller cylinder assembly suitable for use in a high temperature environment, such as a glass fabrication lehr, specifically, a roller cylinder assembly which remains freely rotatable at such elevated temperatures. It is another object of certain particularly preferred embodiments of the invention to provide a glass fabrication system employing multiple such roller cylinder assemblies which cooperate with each other to properly position a hot glass sheet in a horizontal plane of travel above a hot press forming tool. These and other objects and advantages of the invention will be better understood from the disclosure and discussion of the invention set forth below.